(1) Field of the Invention
This invention relates to an improved dashpot system and more particularly to a dashpot for a power cylinder.
(2) Description of the Prior Art
Existing power cylinders have dashpots at the end of their power strokes. A dashpot is a device used to cushion or dampen a movement in a power cylinder and consists essentially of a power cylinder containing a gas or a liquid fluid and a piston moving in the assembly. The end of the power cylinder is configured to contain a centered cup with a tapered inside diameter and the piston is configured to include a centrally oriented cone extending from each side of the piston. The end of the power stroke will be such that the cone will enter the cup and displace the fluid contained therein. However, as the cone penetrates the tapered cup, the radial clearance between the cup and the cone will narrow. As a result, due to the decreased clearance, an increasingly greater resistance to fluid flow develops. This results in a higher pressure being developed in the cup which eventually stops the piston. The pressure results in a higher and higher force being developed and acting on the cone and, thus, on the piston, to decelerate the piston from the direction in which it is moving. In the perfect system, the decelerating counter action of the cup and cone on the piston is of such a force so as to stop the piston. The momentum of the system has, therefore, been counterbalanced and the piston stops without any end position shock.
This arrangement works fine when it is operated with a relatively incompressible fluid, such as an oil. However, when the fluid is a gas, such as air, it will not work because the air compresses. One reason for this is that if the supply for an incompressible fluid is shut off, pressure in the power cylinder drops very rapidly. However, if the supply is shut off for a compressible fluid, then the gas which is already in the power cylinder will simply expand and continue to exert force on the system's piston. In addition, it is necessary to closely control the clearance between the cup and cone in a pneumatic dashpot or insufficient pressure will be built up in the cup to properly decelerate the piston. These close tolerances introduce the risk of the cup and cone having a mechanical impact upon the system reaching the end of the stroke. Further, even if the mechanical components are aligned and are machined perfectly, the trapped air is compressible and therefore does not develop a significant counter-balancing force until a significant portion of dashpot travel has occurred. As a result of the air's compressibility, it is necessary to make pneumatic dashpots lengthy. The combination of the requirements for precise machining, rigid alignment, and a lengthy dashpot results in expensive dashpots which are very susceptible to malfunction due to wear, dirt particles in a system or the like.
Past consideration has been given to combining the benefits of a hydraulic dashpot in a pneumatic power cylinder. However, designing a hydraulic dashpot and incorporating it into caps of a pneumatic power cylinder would be accomplished at considerable risk as such operation would be dependent upon proper alignment of a seal located between the dashpot actuating rod which would be necessary in order to separate the pneumatics from the hydraulics. While leakage of oil into the pneumatic system could cause autoignition, leakage of air into the oil dashpot would destroy the effectiveness of the dashpot. Another alternative is to design a slave hydraulic cylinder which can be connected in parallel to a working pneumatic cylinder. This may eliminate the chance of the hydraulics and pneumatics mixing, but would be accomplished at considerable cost and space.
A search has been conducted of the United States Patent Office records and certain patents have been cited as further illustrative of the art. These patents are: Ban et al., entitled "Piston Unit", U.S. Pat. No. 4,342,884; Hasche et al., entitled "Valve Control Mechanism", U.S. Pat. No. 2,737,970; Williams, entitled "Rod Operated Valve", U.S. Pat. No. 4,359,204; Whittaker et al., entitled "Valve Assembly", U.S. Pat. No. 3,286,465; and Fornasari, entitled "Four-Way Electrovalve Governed By A Thermoactuator Associated With A Thermistor (PTC)", U.S. Pat. No. 4,753,271.